High-nitrogen compounds form a unique class of energetic materials deriving most of their energy from their very high positive heats of formation rather than from oxidation of the carbon backbone, as with traditional energetic materials. The high nitrogen content typically leads to high densities, and the low amount of hydrogen and carbon also allows for a good oxygen balance to be achieved more easily. Oxygen balance is a measure of the oxygen/fuel ratio in a compound. It has been previously demonstrated that high-nitrogen materials can show remarkable insensitivity to electrostatic discharge, friction, and impact.
Tetrazine rings linked by an azo group are practically nonexistent. The only synthesis of azo-1,2,4,5-tetrazines in the literature was reported by Russian scientists in 1971 and 1990. Although they describe the preparation of 3,3xe2x80x2-azobis(6-phenyl-1,2,4,5-tetrazine) and 3,3xe2x80x2-azobis[6-(4-chlorophenyl)-1,2,4,5-tetrazine], no physical properties or proof of structure were given for the compounds.
In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention includes as a composition of matter, the compound of the formula 
where a, b, c, d and e are 0 or 1 and a+b+c+d+e is from 0 to 5. The present invention further includes the species, 3,3xe2x80x2-azobis(6-amino-1,2,4,5-tetrazine), where a, b, c, d and e of the formula are each 0.
The present invention further includes a process of preparing 3,3xe2x80x2-azobis(6-amino-1,2,4,5-tetrazine) including reacting 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine with hydrazine to form a first intermediate product, reacting said first intermediate product with N-bromosuccinimide in acetonitrile to form a second intermediate product, reacting said second intermediate product with ammonia in DMSO to form a 3,3xe2x80x2-azobis(6-amino-1,2,4,5-tetrazine) DMSO solvate, and converting said 3,3xe2x80x2-azobis(6-amino-1,2,4,5-tetrazine) DMSO solvate to 3,3xe2x80x2-azobis(6-amino-1,2,4,5-tetrazine).
Still further, the present invention includes a process of preparing a compound of the formula 
where a, b, c, d and e are 0 or 1 and a+b+c+d+e is from about 0.1 up to 5, comprising reacting 3,3xe2x80x2-azobis(6-amino-1,2,4,5-tetrazine) with an oxidizing agent for time and at temperatures sufficient to form said compound.
The present invention is concerned with the preparation of azo-1,2,4,5-tetrazines and in particular compounds of the formula 
where a, b, c, d and e are 0 or 1 and a+b+c+d+e is from 0 to 5, and especially the compound 3,3xe2x80x2-azobis(6-amino-1,2,4,5-tetrazine) where a, b, c, d and e are 0. In one preferred embodiment of the compound, a+b+c+d+e is from about 3.0 to about 4.0 for ease of preparation.
Interest in the synthesis of azo-1,2,4,5-tetrazines followed from previous studies on azo-1,2,5-oxadiazoles. It was found that 327 kJmolxe2x88x921 of energy is gained in the transformation of 4,4xe2x80x2-hydrazobis-(1,2,5-oxadiazol-3-amine) to 4,4xe2x80x2-azobis(1,2,5-oxadiazol-3-amine). The latter material is a thermally stable, insensitive explosive. Extrapolating from these data, 3,3xe2x80x2-azobis(6-amino-1,2,4,5-tetrazine) would give an even higher heat of formation due to the intrinsically large heat of formation of the 1,2,4,5-tetrazine ring.
Previously it has been shown that oxidation of 3-amino-1,2,4,5-tetrazines leads to the formation of N-oxides in which the oxide moiety is a to the amino group. In the case of 3,6-diamino-1,2,4,5-tetrazine (R=NH2), N-oxide groups are formed at the 1- and 4-positions, whereas with 3-amino-1,2,4,5-tetrazines (R=H), N-oxidation occurs at the two ring nitrogen atoms xcex1 to a nitro group when 3,6-diamino-1,2,4,5-tetrazine is the substrate. Under no circumstances has the formation of an azo or azoxy linkage been observed. Thus it was recognized that the formation of the azo group must be accomplished by a different synthetic approach. Here, synthesis of 3,3xe2x80x2-azobis(6-amino-1,2,4,5-tetrazine) is described, as well as some of the properties of this high-nitrogen material.
The preparation of 3,3xe2x80x2-azobis(6-amino-1,2,4,5-tetrazine) was as follows. The 3,5-dimethylpyrazol-1-yl moieties of 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine have been shown to be good leaving groups in nucleophilic displacements on 1,2,4,5-tetrazines. With readily available 3,6-bis(3,5-dimethylpyrazol-1-yl)1,2,4,5-tetrazine as a starting material, a hydrazo compound was prepared by treatment of the 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine with 0.5 equivalents of hydrazine. Surprisingly, it was found that a variety of oxidizing reagents typically used to oxidize a hydrazo group to an azo moiety (oxidizing reagents such as Br2, NO2, MnO2, HgO, and HONO) did not lead to the formation of the azo group. Oxidation was only achieved with N-bromosuccinimide (NBS), which also brominated the 3,5-dimethylpyrazol-1-yl rings to give an azo compound. The formation of the azo group was confirmed by the absence of signals and stretches for NH in the H NMR and IR spectra as well as elemental analysis. Treatment of the azo compound with ammonia in acetonitrile yielded a precipitate, which upon analysis showed that complete displacement of the 4-bromo-3,5-dimethylpyrazol-1-yl groups did not occur. However, when the reaction was conducted in dimethyl sulfoxide (DMSO) followed by treatment of the reaction mixture with 2-propanol, a red-brown precipitate was isolated.
According to NMR spectroscopy this material was the bis-DMSO solvate of 3,3xe2x80x2-azobis(6-amino-1,2,4,5-tetrazine); however, elemental analysis was inconsistent with the proposed structure, as is typical of compounds high in nitrogen. An X-ray crystal structure analysis of this material confirmed the structure to be the bis-DMSO solvate of 3,3xe2x80x2-azobis(6-amino-1,2,4,5-tetrazine), thus providing the evidence for the synthesis of an azo-1,2,4,5-tetrazine. A density of 1.526 g/cm was determined from the X-ray crystal structure. The molecules are in an E configuration (trans configuration) and form planar sheets despite the presence of DMSO molecules in the crystal. It was believed that this graphite-like structure would lead to a high density of the neat material. Indeed, a gas pyconmetry density of 1.78 g/cm was determined for pure 3,3xe2x80x2-azobis(6-amino-1,2,4,5-tetrazine), which is of a level equal to or greater than the most dense C,H,N molecule known (1.738 g/cm for 5,5-bi-1H-tetrazole).
The DMSO solvate was easily broken by treatment with boiling water to give pure 3,3xe2x80x2-azobis(6-amino-1,2,4,5-tetrazine). The pure material was found to be thermally stable up to 252xc2x0 C. (differential scanning calorimetry), and the heat of formation was measured to be +862 kJmolxe2x88x921 by combustion calorimetry. This is a very high heat of formation and when normalized to a per atom value, a value of 43.1 kJ per atom is realized. Some sensitivity properties include a drop weight impact value of 70 cm (the value for the high-explosive HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) is 25 cm), despite the fact that there are no oxygen atoms in the molecule. The compound was also found insensitive to initiation by spark (0.36 J) or friction (BAM,  greater than 36 kg).
Following preparation of the 3,3xe2x80x2-azobis(6-amino-1,2,4,5-tetrazine), it can be oxidized with various oxygen transfer reagents to form a compound of the formula 
where a, b, c, d and e are 0 or 1 and a+b+c+d+e is from about 0.1 up to 5. In the formula, a+b+c+d+e is preferably greater than about 3. Repeated oxidation steps may be necessary to obtain values greater than about 3.2. In the shown structure, the oxygen to nitrogen bonds are dative bonds and thus no charges are shown. Oxidation can be accomplished by reaction of the 3,3xe2x80x2-azobis(6-amino-1,2,4,5-tetrazine) with trifluoroacetic anhydride in a heterogeneous mixture of hydrogen peroxide in methylene chloride. Other oxidizing may be used as well.